Most cotton and synthetic fiber staple yarns are sized prior to weaving, knitting or other manufacturing technique. Various types of natural and/or synthetic polymers are used for sizing to protect the fibers and yarns from the usual abrasion against the manufacturing equipment or other fibers/yarns. Such abrasion, in the absence of protective sizing, tends to cause various types of damage, such as breakage, pulling, pilling, and the like. Such damage is especially problematic when the fibers/yarns are subject to high speed processing, since otherwise the processing units need to be stopped to remove and/or repair damaged fibers and yarns.
Even if the fibers or yarns are not broken, other types of damage occurring during fabric formation could result in non-uniformities in the fabric. Such nonuniformities can lead, in turn, to uneven dyeing and finishing operations, thereby reducing the value of the resulting fabric.
Sizes for yarns made from natural fibers such as cotton generally are largely based on natural polymers and their derivatives, e.g., starches, various types of modified starches, and cellulose derivatives. With the advent of synthetic fibers and synthetic fiber containing fabrics (e.g., polyesters, polyacrylates, polyamides, polyarylamides and the like), synthetic polymers have been employed as the sizing agents for yarns. Examples of such synthetic polymers include, polyvinyl alcohols, partially esterified poly(vinylacetate), polyesters and others. Representative sizing agents for yarns are shown in the following Table 1.
TABLE 1Polymeric Sizing Agents Used In Textile ProcessingNatural PolymerSynthetic PolymersStarchesPoly(vinyl alcohol)Modified or Refined StarchesPoly(vinyl acetate)Starch DerivativesAcrylicsCellulose DerivativesSulfonated PolyestersPolyurethanesStyrene Copolymers
Requirements for polymers used for sizing may vary from one type of yarn to another, depending on such things as fiber content, manufacturing processes to be used, anticipated downstream process operations, yarn type, etc. However, useful polymers must generally be a good film-former with abrasion resistance and flexibility. Useful polymers will also typically have one or more of the following properties: compatibility with other ingredients in the formulation; non-corrosive to mill equipment; low foaming; easy removability from the yarns; and relatively low viscosity to allow uniform application to the yarns.
Typical sizing formulations may also have to maintain their stability for many hours at elevated temperatures and contain % solids from as little as 3% to over 20% to achieve equal add-on to sized yarns after drying. A variety of additives, such as antifoam agents, waxes, lubricants are present in representative formulations.
In addition to the film-forming polymer, size mixes generally include lubricant and various other additives. Lubricant wax is added to increase abrasion resistance of the yarn which is especially useful for rapiers and projectile machines. Lubricants with anti-sticking agents, e.g., lecithin, also prevent sticking of the film-forming size polymer, e.g., polyvinyl alcohol (PVA), to dry cans. Emulsifiers are often added to improve solubility. In the case where wax is the lubricant used, paraffin or marine glycerides may be added to harden the wax and better lubricate the yarns. A problem with addition of wax or oil lubricants to size mixes is that not all of the wax or oil may be removed properly during scouring, even when caustic is used for the scour; any remaining wax or oil lubricant may interfere with subsequent operations, including, for example, heat setting (smoke or fumes) and dye stains in dyeing. Nevertheless, the use of lubricant, primarily waxes, in textile sizes has been essentially universally adopted. A variety of additives, such as antifoam agents, waxes, lubricants may also be present in representative formulations.
Conventional processes for sizing warp yarns, are conducted in machines called “slashers”. In a slasher, sheets of warp yarns move from a battery of beam creels through a container that contains the sizing formulation. The wetted yarns are subsequently squeezed of excess liquid polymer (wet split), then passed through a series of heated cylinders to dry the warp sheets that are then wound up on a beam for subsequent use in weaving or knitting or other fabric formation process. In some cases, yarns may be sized individually (single end sizing) and are then usually subsequently recombined during, for example, winding.